Developer for electrostatic photography



Feb. 11, 1958 J. D. SCHROEDER 2,822,779

DEVELOPER FOR ELECTROSTATIC PHOTOGRAPHY Filed Sept. 24, 1956 IN VEN TOR.

United States Patent F DEVELOPER FOR ELECTROSTATIC PHOTOGRAPHY John D. Schroeder, Tulsa, Okla., assignor, by mesne assignments, to Century Geophysical Corporation, Tulsa, Olden, a corporation of Delaware Application September 24, 1956, Serial No. 611,693

4 Claims. (Cl. 118-637) This invention relates to an improved apparatus for use in electrostatic photography for developing a visual image on a sheet carrying a latent electrostatic image.

Specifically, the present invention relates to an improved magnetic-brush apparatus for developing electrostatic latent images.

Electrophotographic processes involve converting a light image into an electrostatic charge pattern on a suitable insulating layer, usually a specially prepared paper coated with metallic oxide particles suspended in a resin binder. Development of such an electrostatic charge pattern into a visual image is usually accomplished by brushing the charged surface with a dyed thermoplastic powder commonly called toner. The powder will be differentially attracted to the various parts of the charged surface and will thus define on the surface a visible pattern corresponding to the latent electrostatic image.

Various means have been proposed for systematically brushing the developing powder over the surface of the charged paper. One of the most widely used of these involves a so-called magnetic brush, consisting of a series of elliptical washers spacedly mounted on a shaft at an angle. When such a brush is magnetized and coated with toner, and is then rotated over the surface of the photosensitive sheet, the washers effectively wipe the toner back and forth across the surface as the shaft of the brush is rotated. If the photosensitive paper be continuously advanced in a direction perpendicular to the axis of rotation of the brush, the entire surface of the paper will be progressively brushed with toner powder.

For good developing the peripheral speed of the washers forming the magnetic brush must be several times the speed at which the photosensitive sheet is passed by the brush. At the same time, the centrifugal forces gen erated by high rotational speed of the magnetic brush will, if the speed be excessive, cause the toner powder to leave the brush, so that the maximum practicable brush speed is relatively low. These factors have, in the prior art, imposed a decided limitation on the speed with which electrophotographic images can be developed.

The major object of the present invention is to provide a magnetic-brush developer for use in electrophotography which will permit developing of electrostatic images at a speed several times as great as has been practicable in the past.

In the achievement of this major object, my invention involves as another object the provision of a novel magnetic brush which will, for a given speed of rotation, brush toner powder across the surface being developed at a speed several times faster than pri0r-art magnetic brushes.

Another object of the present invention is to provide an improved magnetic brush in which greatly increased brushing speed is achieved without increasing the centrifugal forces operating on the toner powder.

A still further object of the present invention is to provide an improved magnetic brush which makes possible, for any given speed of brush rotation, a rate of develop- 2,822,779 Patented Feb. 11, 1958 ment several times as great as prior-art magnetic brushes could achieve.

Other objects and advantages of the present invention will appear from the following description of a typical embodiment thereof.

In the appended drawing, Figure 1 is a view in vertical section of a typical magnetic-brush developer embodying the principles of my invention. Fig. 2 is a fragmentary plan view of the Fig. 1 brush, bringing out the structural details of my novel brush and showing how it operates to brush toner powder rapidly over the entire surface of a photosensitive sheet to be developed.

As may be seen from Figs. 1 and 2, my magnetic brush consistsof a large number of ,radially'corrugated magnetizable washers 11 spacedly mounted on a' shaft or mandrel 12. The radial corrugations in the washers 11 distort the contour of the washers by an axial distance equal to or exceeding the washer thickness, and the washers are stacked on the mandrel in such manner that the corrugations register with one another in the axial direction. Rather than being closely stacked together, however, the individual washers 11 are spaced one from another on the mandrel, as is brought out clearly in Fig. 2, providing a .slight overlap, in the axial direction, between any given washer and its neighboring washers. That is, the washers are so stacked on the mandrel 12 that aplane tangent to one washer and perpendicular to the shaft willinterceptthe neighboring washer.

If desired, the central aperturesin thewashers :l-l-may be made slightly elliptical, and the washers be correspondingly fixed in position on the mandrel so that their medial planes define a slightly acute angle with respect to the mandrel axis.

Figs. 1 and 2 bring out the manner in which my magnetic brush is used in connection with the photosensitive paper and a supply of toner powder. The toner powder 30 is carried in a suitable pan-like container 31 mounted beneath the brush, the brush being normally mounted with the axis of shaft 12 horizontal. The paper 40 to be developed is progressively advanced past the brush, a charged plate 41 preferably being mounted on the opposite side of the paper from the brush to facilitate deposit of the toner powder on the partially discharged areas of the paper.

The brush assembly, consisting of the shaft 12 and the washers 11, is magnetized, preferably by suitably positioned permanent magnets (not shown). When shaft 12 is rotated in any desired manner, the magnetized washers 11 pick up the toner powder 30 from pan 31 and cause it to be wiped back and forth across the surface of the paper 40.

The foregoing description of the operation of my invention is typical of all magnetic-brush developers and does not per se form a part of my invention.

The novel structural aspects of my invention lie in the formation and positioning of the washers 11, and the novel operational feature of my invention lies in the fact that with my magnetic brush the toner powder is wiped back and forth on the face of paper 40 several times for each revolution of shaft 12.

This represents a most important improvement over prior-art magnetic brushes, which achieved only one cycle of wiping action for each rotation of the shaft.

With my invention, it is possible to obtain six or more complete wiping cycles for each rotation of the brush shaft, the exact number depending on the number and spacing of the radial corrugations in the Washers 11. In the particular structure shown in the drawing, a total of six wiping cycles will occur for each revolution of shaft I have discovered that the effectiveness of development is largely governed by the rate at which the powder is wiped across the face of the paperyand, by the use of my improved brush design, I avoid entirely any tendency on the part of my brush to throw the toner powder around uncontrolledly. With my brush, excellent developing action can be achieved with comparatively low shaft speed, with the result that the centrifugal forces operating on' the toner powder are at all times quite low and hence have no tendency to make the powder fly about; 7

Due to the greatly increased wiping speed achieved with my magnetic brush, the paper 40 can be advanced much more rapidly than in prior-art apparatus without sacrificing in any respect the quality of the developed image. .While I have in this specification described in considerable .detail a typical embodiment of my invention, it will be understood that persons skilled in the' art may make numerous changes in'and departures from the disclosed structure within the spirit of my invention. It is therefore" my desire that the scope of my invention be determined primarily with reference to the appended claims.

1. In an apparatus for developing a visual image on a sheet carrying a latent electrostatic image, the combination which comprises a source of finely divided toner powder, a rotatable shaft of magnetizable material carrying 2. The combination of claim 1 in which the plane of each said washer is axially distorted by said corrugations by an amount at least equal to the thickness of such washer.

3. In an apparatus for developing a visual image on a sheet carrying a latent electrostatic image, the combination which comprises a source of finely divided toner powder, a rotatable shaft carrying axially secured thereto a plurality of spaced washers, said shaft and washers being formed of magnetizable material, means for magnetizing said shaft and washers, means for rotating said shaft, and means for passing said sheet tangentially past the edges of said washers while said shaft is rotating, each of said washers having a plurality of radial corrugations therein, whereby the point of contact between the edge of any such Washer and a plane surface tangent thereto will oscillate a plurality of times during a single rotation of said shaft, said powder source being disposed near said shaft and washers for depositing said toner powder thereon during rotation of said shaft.

4. The combination of claim 3 wherein the plane of each of said washers is axially distorted by said corrugations by an amount at least equal to the thickness of such washer.

3 References Cited in the file of this patent g UNITED STATES PATENTS 

